Staphylococcal Protein A (SPA) is a protein that is found in nature anchored to the outer membrane of the gram-positive Staphylococcus aureus bacterium, the organism which is commonly associated with medically significant human “Staph” infections. The role of SPA in the life cycle of S. aureus remains uncertain, but some studies have correlated the presence of SPA with pathogenicity of the organism.
Functionally, SPA is well known for its ability to tightly, but reversibly, bind to the constant region of an immunoglobulin molecule (IgG). This property has been widely exploited in the affinity purification of antibodies for commercial uses. For example, SPA can be purified from S. aureus and covalently bound to various forms of solid supports to thus immobilize it to make an affinity chromatography resin. Crude preparations of antibodies can then be passed over such an immobilized SPA resin to bind and capture the commercially valuable antibody, while contaminating materials are washed away. The bound antibody may then be eluted in pure form by a simple adjustment of the pH.